A thermal flow sensor measures fluid velocity of, such as, gas and liquid utilizing the principle of balancing input and output energy. In a balanced system of a thermal flow sensor, the input is simply thermal energy converted into electric format and the output thermal energy includes thermal conductivity, thermal radiation and convective heat losses. Among them, the convection heat loss is the dominating term. When the input energy and the output energy are balanced, the convective heat-loss is used to measure the velocity of the fluid.
FIG. 1 illustrates the system diagram of a conventional thermal flow sensor. As shown in the figure, a thermal flow sensor comprises: a fluid channel 1 (see FIG. 3) through which fluid F flows, a fluid temperature sensor 6 to sense the temperature of the fluid F, a heater 72 to supply thermal energy to the fluid F, a DC power supply 9 to supply electric current to heater 72, a heater temperature sensor 74 to sense the temperature of the heater 72, a Wheatstone bridge module 10 to regulate the DC current supplied to the heater 72 and a microprocessor 11 to measure changes in the voltage of the current supplied to the heater 72 and to convert said change into velocity values of the fluid F.
In order to shorten the response time, a thermal flow sensor operates under a "constant temperature difference mode". Under such mode, heater 72 is positioned inside the fluid channel 1 (see FIG. 3), and a constant temperature difference between heater 72 and the fluid F is maintained, such that the temperature of the heater 72 is always higher than that of the fluid F. In doing this, temperature information obtained from the fluid temperature sensor 6 and the heater temperature sensor 72 may be taken for reference. In order to transfer thermal energy from the heater 72 to the fluid F, heater 72 is always in direct contact to the fluid F. When the fluid flows, due to the temperature difference between the heater and the fluid, thermal energy contained in the heater is carried away by the fluid. The thermal energy carried away by the fluid is in direct proportion to the square root of velocity of the fluid. To maintain a constant temperature difference, a feedback control loop is used to regulate electrical current supplied to heater 72. When the constant temperature difference is maintained, the thermal energy (electric currency) needed to retrieve the temperature of the heater 72 may be measured and the fluid velocity may be known.
In the conventional art, the said feedback control loop may be a Wheatstone bridge module. FIG. 2 illustrates the circuit diagram of a Wheatstone bridge module for a thermal flow sensor. As shown in the figure, a Wheatstone bridge module 10 has a Wheatstone bridge 21, an operational amplifier 22 and a heater R.sub.H. At one branch of the Wheatstone bridge there are fluid temperature sensor R.sub.f and resisters R.sub.1 and R.sub.2, while at the other branch are heater temperature sensor R.sub.5 and resistor R.sub.3. Temperature sensor R.sub.f and resisters R.sub.1 and R.sub.2 provide a reference voltage V.sub.1 to the operation amplifier 22 and heater temperature sensor R.sub.5 and resistor R.sub.3 Provide another reference V.sub.1 to the operation amplifier 22. By setting up a minute voltage difference between V.sub.1 and V.sub.2, i.e., with the assistance of resistor R.sub.1, the circuit will come to a steady state by heating up resistor R.sub.H (the heater temperature sensor 74) and resistor R.sub.5 (fluid temperature sensor).
In order to improve the overall performance of a thermal flow sensor system, it is necessary to enhance the ratio of the heat convection loss, in contrast to the radiational and conductive losses of the thermal energy, during the flow of the fluid. Over the past years various improvements for the thermal flow sensor have been invented. Among these improvements, some used silicon micro-machining technology to minimize the size of the heater 72 or to isolate the heater 72 from the rest parts of the flow sensor, so to minimize unwanted losses of thermal energy during the flow of the fluid. Some proposed to position the heater temperature sensor 74 as close as possible to the heater 72 by using concrete elements such as multi-layer, thin film resistors positioned on a microbridge or on a micro membrane.
Other developments to the performance of the thermal flow sensor include: U.S. Pat. No. 4,326,412, to Hiroshi et al. (1982), U.S. Pat. No. 5,186,051, to Stecher et al. (1996) and U.S. Pat. No. 5,623,097, to Horiguchi et al. The Hiroshi invention disclosed a thermal flow sensor used in fuel control system for auto combustion engine. In the Hiroshi invention, a network of metal wire segments in two cross sectional planes in a fluid pipe is used to measure a non-uniform flow of gasoline in an average manner. In the flow, adjacent to the boundary the dragging effect slows down the velocity of the gasoline. In the Stecher invention, disclosed are a thin film heater and a heater temperature sensor coupled very close to each other in order to provide precise measurement of the heater temperature. In the Horiguchi invention disclosed is a heater with a sensor stacked to it and positioned on a microbridge. These and other improvements, however, have a common problem. That is, the heater and temperature sensor are positioned in parallel to the fluid flow. This arrangement creates a thermal boundary layer on the surface of the heater-temperature assembly. The thermal boundary layer impedes the heat convection efficiency from the heater to the fluid. Other problems include that the components used in these disclosures are not compact enough and make the device bulky.
In the conventional art, the fluid temperature sensor 6 is positioned in the up-stream direction of the flow and the heater 72 in the down-stream direction. This creates a problem in installing the flow sensor while the direction of the flow shall be taken for consideration.
It is thus a need in the industry to provide an integrated flow sensor with higher precision. It is also a need to provide a thermal flow sensor where the area of the thermal boundary layer may be minimized. It is also a need to provide a thermal flow sensor that is easy to manufacture under semiconductor manufacture process, so that the heat capacity of the system may be reduced. It is also necessary to provide a thermal flow sensor where the factor of flow direction may be ignored.